Yes, a M.2 NVMe drive qualifies as a PCIe SSD.
Yes, a M.2 NVMe drive qualifies as a PCIe SSD.
You've already installed a M.2 SSD in the top slot, and it's disabling two other slots—this is working as expected. The middle slot isn't affecting anything further, which is normal. There’s no indication in the system that changing slots will impact performance beyond what you're seeing.
Even with all three slots filled, it shouldn't impact your graphics card, since it's typically installed in the top position. The lower slot is generally reserved for devices like sound cards, Wi-Fi cards, or USB hubs, which usually don't require high speeds.
There are several M.2 devices that rely on the SATA protocol. M.2 represents a compact PCIe interface, while SATA is a standard communication protocol used for data transfer.